α-Photooxygenation of chiral aldehydes with singlet oxygen

Abstract

Organocatalytic α-oxygenation of chiral aldehydes with photochemically generated singlet oxygen allows synthesizing chiral 3-substituted 1,2-diols. Stereochemical results indicate that the reaction in the presence of diarylprolinol silyl ethers is highly diastereoselective and that the configuration of a newly created stereocenter at the α-position depends predominantly on the catalyst structure. The absolute configuration of chiral 1,2-diols has been unambiguously established based on electronic circular dichroism (ECD) and TD-DFT methods.

Recent applications of porphyrins as photocatalysts in organic synthesis: batch and continuous flow approaches

“…Singlet oxygen has also been efficiently used for enantioselective and chemoselective oxidations of many organic compounds. Notably, the Gryko's group recently described an enantio-and diastereoselective approach involving a porphyrinbased photooxygenation of aldehydes with sequential reduction to yield chiral diols in yields up to 91% and significant er (up to 96:4), but low dr (up to 66:33) (Scheme 40) [90].…”

Stereoselective [4+2] Cycloaddition of Singlet Oxygen to Naphthalenes Controlled by Carbohydrates

“…However, the enantioselective transfer of 1 O 2 is still challenging, because of its short lifetime. Although the α-hydroxylation of aldehydes and ketones was realized by organocatalysis with moderate to good ee ( enantiomeric excess ) [ 14 , 15 , 16 ], ene reactions or cycloadditions proceed less successfully. Thus, asymmetrically modified zeolites [ 17 ], chiral salts [ 18 ], or chiral sensitizers [ 19 ] afforded only low stereoselectivities.…”

Visible Light-Promoted Enantioselective Aerobic Hydroxylation of β-Ketocarbonyls by Chiral Primary Amine Catalysis

“…5 Unfortunately, the reactions showed only moderate enantioselectivity, and extensive efforts on further improvements have met with only very limited success over the past two decades. 6 The associated difficulties are multifaced. First, the stereocontrol with singlet oxygen is notoriously challenging due to the fleeting nature and small size of 1 O 2.…”

“…Previous studies by Córdova have shown that aminocatalysis was viable to mediate aerobic hydroxylation of ketone and aldehyde by enamine-singlet oxygen coupling . Unfortunately, the reactions showed only moderate enantioselectivity, and extensive efforts on further improvements have met with only very limited success over the past two decades . The associated difficulties are multifaced.…”